Rosin esters, i.e., esters of rosin and polyols, have been known for almost a century (see, e.g., U.S. Pat. No. 1,696,337) and are widely used as tackifiers for packaging and pressure-sensitive adhesives.
The simplest rosin esters are made by reacting a rosin acid, which may be disproportionated, with one or more polyols such as glycerol, pentaerythritol, and diethylene glycol. Improvements relate to ways to make rosin esters with low color or low odor. For examples, see U.S. Pat. Nos. 4,477,613; 4,758,379; 5,504,152; 5,830,992; and 5,969,092.
In another common approach, a rosin acid is first heated with an enophile such as an unsaturated dicarboxylic acid or anhydride (e.g., maleic anhydride or fumaric acid) to form an adduct by an ene reaction or a Diels-Alder reaction. Acid groups in the adduct are then esterified by reaction with a polyol (e.g., glycerol, pentaerythritol). For examples of rosin esters made this way, see U.S. Pat. Nos. 4,201,701; 4,643,848; 5,559,206; 5,691,405; 6,171,385; and 7,994,106, and U.S. Pat. Appl. Publ. No. 2011/0213120.
Occasionally, a one-pot approach is used in which a rosin acid, polyol, and unsaturated diacid or anhydride are heated together. This usually involves principally esterification rather than formation of an adducted resin (see, e.g., U.S. Pat. No. 4,201,701).
In yet another approach, a rosin ester is made in the usual way be reacting a rosin acid and a polyol or mixture of polyols. In a second step, an adducted resin is then made by heating the rosin ester with an unsaturated diacid or anhydride. Usually, the acid groups are then modified to enhance water solubility or water dispersability. For examples, see U.S. Pat. Nos. 4,751,025 and 5,504,129.
Rosin esters have been used to make surfactants or emulsifiers. For instance, U.S. Pat. No. 4,260,550 teaches to form an adduct from rosin and maleic anhydride, followed by esterification with polyethylene glycol, 6000 mol. wt. (see Examples VIII-X). The PEG-modified rosin ester is used as a surfactant for making a hair care product. In another example, U.S. Pat. No. 6,274,657 teaches the preparation of a surfactant by reacting rosin or dimerized rosin with polyethylene glycol, 8000 mol. wt. The surfactant is combined with rosin ester tackifiers to make stable aqueous dispersions. In U.S. Pat. Nos. 6,900,274 and 7,452,941, a surfactant is made by reacting maleic anhydride with a terpene or hydrocarbon resin to form an adduct, which is further reacted with polyethylene glycol, 2000 mol. wt. This surfactant is also useful for making stable aqueous dispersions comprising rosin ester tackifiers. U.S. Pat. Appl. Publ. No. 2007/0135542 describes emulsifiers made by reacting a rosin with polyethylene glycol, followed by reaction with polyphosphoric acid. The resulting water-dispersible product is combined with tackifiers to give aqueous dispersions useful for pressure-sensitive adhesives.
In another variation on the surfactant theme, U.S. Pat. No. 5,552,519 teaches to prepare rosin ester surfactants in three steps. In one aspect, a rosin is first esterified with a polyol (e.g., glycerol) to an acid value of less than 30 mg KOH/g. The rosin ester is then grafted with an unsaturated dicarboxylic acid or anhydride. The grafted rosin ester is esterified with polyethylene glycol having a weight-average molecular weight (Mw) of 4000 to 16000. This material is used as a surfactant for creating an aqueous dispersion of a rosin ester based on rosin and pentaerythritol. The dispersion is combined with a tackifier and an acrylic polymer, and this mixture is used to make a pressure-sensitive adhesive.
Improved tackifiers are needed, particularly ones useful in pressure-sensitive adhesives prepared from aqueous dispersions. In particular, the industry needs tackifiers than can be combined with acrylic emulsions to give adhesives having good loop tack and peel properties. Preferably, the tackifiers could be made using conventional, economical starting materials, conventional equipment, and ordinary process steps. Ideally, the tackifiers could reduce reliance on surfactants and deliver desirable adhesive properties while reducing coat weight by 10-20% or more.